8. Coherent X-ray diffraction imaging of biological nano-crystals

X-ray crystallography has been proven to be an extremely efficient investigation method to solve the structure of inorganic and organic matter at the atomic scale. Although several methods have been employed to circumvent the intrinsic phase problem other limitations exist for classical x-ray crystallographic methods. One severe limitation for protein structure determination using crystallography, for example, is the availability of large crystals of sufficiently high quality.Coherent X-ray Diffraction Imaging (CXDI) is a recently developed method, which can in principle overcome the restrictions mentioned above. It relies on the fact that the coherent diffraction pattern, which is generated when a specimen is illuminated with coherent light, resembles the Fourier transform of the electron density distribution in the object. Instead of relying on a lens, CXDI uses novel computer phase retrieval algorithms to reconstruct the object’s electron density map from the measurements of the coherent far-field diffraction patterns.While at future X-Ray Free-Electron Laser (XFEL) sources CXDI may ultimately yield subnanometer resolution of single macromolecules currently existing highly brilliant third-generation xray synchrotron sources already provide sufficient coherent flux for investigations of micron-sized specimens. For example, we have recently shown that a combination of coherent diffractive imaging with a scanning approach yields high-resolution phase and amplitude images and potentially extends the currently achievable resolution of direct x-ray radiographic or tomographic methods into the nanometer range [1]. Currently this technique is being further developed, in particular with respect to its application for cells and cellular substructures. Scientifically as important and desirable is the application of CXDI to protein nano-crystals or clusters of a few tens or hundreds of macromolecules.